The invention relates to a process for producing a single-core or multi-core superconducting wire with application of a metal molybdenum chalcogenide (Chevrel-phase) in the superconducting phase, particularly by PbMo.sub.6 S.sub.8, whereby the superconducting phase or the initial components for forming such a phase is/are placed vacuum tight in a molybdenum shell and said shell is advantageously placed in a steel jacket and whereby said unit is subsequently processed by hot shaping to produce a superconducting wire.
The development of superconducting wires and cables using binary compounds (type II superconductors), in particular Nb.sub.3 Sn and NbTi, has made rapid advances in the course of the last few years. The technology of manufacturing single- and multi-filament conductors from superconducting and normally conducting materials arranged spatially next to each other has been constantly improved. The process steps of hot and cold shaping, e.g. extruding and wire drawing, play a significant role in the manufacture of multi filament conductors (German Pat. DE-AS No. 25 16 745; German Pat. DE AS No. 19 45 640). The developments in this field of technology have met upper limits as to three critical superconducting quantities, i.e., temperature, current density and magnetic field, which have not been raised to any appreciable extent.
It has been suggested to raise the limits of at least one of the critical superconducting quantities by changing from binary to ternary superconductive phases. The change to ternary systems, for example in the form of Chevrel phases (metal-Mo-chalcogenides) offers additional possibilities for varying metallurgical parameters important to superconductivity. However, processing problems in connection with ternary systems are becoming more extensive.
Accordingly, the few publications dealing with the feasibility of manufacturing superconductors with the application of Chevrel phases do not offer very much encouragement. In the report "Research Opportunities in Superconductivity"--published at a seminar held on Aug. 22 and 23, 1983 in Copper Mountain, CO., U.S.A., the following was stated on page 16: "Many new materials, e.g. PbMo.sub.6 S.sub.8, cannot be produced by application of the extrusion and wire drawing technologies, which were so successfully used with Nb-Ti and Nb.sub.3 Sn." And in an article dealing with the Chevrel phases as new groups of high field superconductors, which was published in the trade journal "Metall", 35th Year, No, 4, April 1981, the following was stated on page 289, column 3: "Unforturately, the manufacture of wires of ternary molybdenum-chalcogenides with a current bearing capacity that would be of interest technologically has not yet been achieved because of their poor metallurgical properties."
The problems encountered in the processing of Chevrel phases to superconducting cables, and in particular with PbMo.sub.6 S.sub.8 are manifest. The metallic material PbMo.sub.6 S.sub.8 is highly brittle. For this reason, it appears as a powdery heterogeneous phase. In addition to its low ductility, the Chevrel phases exhibit high sensitivity to oxygen and nitrogen, and also to other metallic elements, in particular Fe, Co, Ni and Cr. This means that the Chevrel phases have to be protected in the manufacture of superconducting wires and cables, against both direct contamination from these metals and the interdif- fusion of metals through the sheathing materials at the high processing temperatures required. Like all superconductors, the Chevrel phases in superconducting wires and cables have to withstand mechanical stresses, i.e., they have to maintain the material densities achieved by means of suitable manufacturing procedures even when the conductor is flexed to form coils and when extreme tensile forces and pressures occur due to the high magnetic forces in a superconducting coil. Furthermore, superconducting wires produced with the application of Chevrel phases must be "stabilized cryogenically", i.e., the superconducting phase has to be in good thermal and electrical contact with a sufficient quantity of highly conductive, normally conducting metal which, in the event of a local breakdown of superconductivity, is capable of temporarily conducting the current, and which eliminates Joulean dissipation by transferring it to the helium cooling medium without the occurrence of any inadmissible rise in the temperature of the conductor ("Metall", 35th Year, No. 4, page 292).
Some isolated attempts have been made to manufacture superconducting wires having a Chevrel phase. For example, in a manufacturing test, the superconducting material PbMo.sub.6 S.sub.8 or suitable initial components thereof were loaded vacuum-tight in a molybdenum or tantalum tube. This molybdenum or tantalum tube was subsequently sealed by welding in a tube made of stainless steel in order to protect the inner tube against oxidation during further processing. Further processing was carried out in two steps by hot drawing at 750.degree. C. and 600.degree. C., respectively, with intermediate annealing at 850.degree. C. However, the result was not satisfactory. Critical current densities and magnetic fields were not achieved. It was assumed that the reason for this unsatisfactory result was that processing by high-temperature drawing caused the molybdenum tubes to become brittle and crack and that iron, chromium and/or nickel from the steel jacket diffused into the Chevrel phase through these cracks, which led to a destruction of the Chevrel phase (Seeber, B.; Rossel, C.; Fischer, O.: "Proc. of the International Conference on Ternary Superconductors", Lake Geneva, Wisconsin, USA, 1980; Seeber, B.; Rossel, C.; Fischer, O., Glaetzle, W.: "Investigation of the Properties of PbMo.sub.6 S.sub.8 Powder Processed Wires", in IEEE Transactions on Magnetics, Vol. MAG-19, No. 1, 1983, pp 402-405, in particular page 402 and page 405, left column).
Therefore, the problem the present invention addresses is the manufacture of industrially processible, superconducting wires with the application of superconductive metal-molybdenum chalcogenides (Chevrel phases), with which the drawbacks of the known designs do not appear. Said problem involves the following: During the manufacture of the wire material, the Chevrel phase has to be protected against contamination and interdiffusion of elements which destroy the Chevrel phase. Furthermore, by the selected manufacturing process the Chevrel phase has to be compressed with a density sufficient to achieve a permanent adjustment or setting of the theoretically achievable critical superconductor values. In addition, said manufacturing process to be selected has to supply the structure or pattern of the magnetic flux desired by the expert, and assure the mechanical strength sufficient for further processing to superconducting cables and coils.